Recently, oxide thin films have been usefully employed in displays and semiconductor devices. In particular, a Zinc Oxide (ZnO) is a II-VI group direct transition semiconductor and has a high band gap of 3.37 eV so that it is transparent in a visible light region. Also, the Zinc Oxide (ZnO) has an exciton bond energy of 60 meV and is thus widely employed as an optical element (see D. C. Look et al. “Recent advances in ZnO materials and devices” Materials Science and Engineering: B 80, 383, (2001)).
The Zinc Oxide may have n-type characteristics due to native defects such as interstitial Zinc and oxygen vacancies, and may change electrical properties up to 10−2 to 1010 Ωm depending on process conditions. In this case, the electron concentration may be further increased to use the Zinc Oxide as a transparent electrode, for which III-group elements or VII-group elements are doped thereinto, Gallium (Ga), Aluminum Al), and Indium (In), which belong to the III-group, are representative dopants, and materials doped with Ga, Al, and In are named Gallium Zinc Oxide (GZO) (see Quan-Bao et al. “Structural, electrical, and optical properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering” Journal of Crystal Growth, 304, 64 (2007)), Aluminum Zinc Oxide (AZO) (see Byeong-Yun Oh et al. “Properties of transparent conductive ZnO:Al films prepared by co-sputtering” Journal of Crystal Growth, Volume 274, 453, (2005)), and Indium Zinc Oxide (IZO) (see E. J. Luna-Arredondo et al. “Indium-doped ZnO thin films deposited by the solgel technique” Thin Solid Films, 490, 132 (2005)), respectively. These transparent electrodes have come into the spotlight as materials capable of taking the place of transparent electrodes formed of Indium Tin Oxides (ITO) which have been widely applied to electronic devices in recent years.
The transparency of ZnO enables it to be implemented in a transparent transistor, and the high mobility of ZnO allows it to be used as an active layer of the transistor of a display device. When ZnO is a bulk, it has a superior mobility of about 200 cm2/Vs (see D. C. Look et al. “Electrical properties of bulk ZnO” Solid State Commun. 105, 399 (1998)). In addition, ZnO has an ionic bond so that a mobility difference between single crystalline ZnO and amorphous ZnO is not significant compared to Silicon (Si). Such properties allow it to be applied to modern display devices which require an active layer having high mobility. Furthermore, many other elements have been alloyed with a Zinc compound in order to obtain a higher mobility and a stable active layer. Materials having a greater orbital than the 5s-orbital of Zinc and a larger ion radius than Zinc (e.g., Indium, Tin, Thallium) are added to the Zinc compound to form alloys such as In—Ga—ZnO (IGZO), In—ZnO (IZO), Sn—ZnO (SZO), Sn—Ga—ZnO (SGZO), In—Sn—ZnO (ISZO), Tl—ZnO (TZO), and Tl—Ga—ZnO (TGZO). These material share more outermost electrons of the 5s-orbital due to greater positive ions than Zinc, which contributes to the electron mobility. In this case, Gallium of the active layer contributes to adjustment in electrical properties and enhancement of stability.
Currently, research is being performed particularly on IGZO using vacuum equipment associated with pulse laser deposition (PLD), sputtering, chemical vapor deposition (CVD) and so forth to be applied to various display devices. However, when such vacuum equipment is employed, the unit price of the process is increased due to the price of the larger-sized equipment.